Ionization gauges, more specifically Bayard-Alpert (BA) ionization gauges, are the most common non-magnetic means of measuring very low pressures. The gauges have been widely used worldwide. These gauges were disclosed in 1952 in U.S. Pat. No. 2,605,431, which is herein incorporated by reference in its entirety. A typical ionization gauge includes a cathode filament as an electron source, an anode, and an ion collector electrode. For the BA ionization gauge, the electron source is located outside of an ionization space or an anode volume which is defined by a cylindrical anode screen. The ion collector electrode is disposed within the anode volume. Electrons travel from the electron source to and through the anode, cycle back and forth within the anode, and are consequently retained within the anode.
In their travel, the electrons collide with molecules and atoms of gas that constitute the atmosphere whose pressure is to be measured. This contact between the electrons and the gas creates ions. The ions are attracted to the ion collector electrode, which is typically connected to ground. The pressure of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S) (Iion/Ielectron), where S is a coefficient with the units of 1/Torr and is characteristic of a particular gauge geometry, electrical parameters, and pressure range.
Generally, in existing dual filament hot-cathode ionization gauges, each filament is operated at two different emission currents with the first being about 4 mA and the second current level being about 100 μA. Generally, the ionization gauge is operated to switch between these two current levels based on adjustable preset pressure levels that are manually set by the user. In another system, the emission current varied step-wise through ten levels between minimum and maximum levels specified by a user. The purpose of these multiple current levels is to have the most sensitivity for the ionization gauge in the respective pressure range of operation.